Ambri raises money to build a commercial-scale factory for batteries that promise low-cost, multi-hour energy storage for the grid.

Progress being made by battery startup Ambri suggests that the market for long-duration grid energy storage is finally taking shape. Storing wind and solar power using today’s battery technologies is too expensive, but new technologies could make it affordable, enabling wider use of renewables.

The Cambridge, Massachusetts, company last week said it has raised a $35 million series-C round to fund the production of prototype batteries from an existing factory and finance construction of a commercial-scale plant. The company intends to test prototypes in the field this year and produce full-size batteries for paying customers by 2016.

For Ambri, the investment marks a shift from demonstrating the science behind its technology—a battery cell that uses two liquid metals as electrodes and a salt electrolyte—to engineering a commercially viable product (see “Ambri’s Better Battery”). It also reflects a growing confidence that utilities and renewable energy project developers are willing to invest in new energy storage technologies.

“The need for longer duration storage is starting to materialize,” says Rick Fioravanti, the head of distributed energy resources and consultancy DNV GL Energy. “It is up to the manufacturers to deliver.”

The cost of producing batteries is another important factor in undercutting incumbent battery technologies. Because Ambri’s manufacturing process is simple—it involves placing metal pucks and salt electrolyte material in a four-inch-square stainless steel cell—much of the production can be automated, says David Bradwell, the company’s chief technology officer.

At its existing manufacturing plant in Marlboro, Massachusetts, Ambri assembles its cells with robots normally used in the auto industry. The same kind of robot will be used at the company’s full-scale factory, which will make 130 megawatt-hours of batteries per year, says Bradwell. “We expect to be almost an order of magnitude less in production cost than lithium-ion batteries, based on the elegance of the cell,” he says.

Building a new lithium-ion battery factory costs about $100 million and requires complex processes and machinery, says Andrew Chung, a venture capitalist at Khosla Ventures and an Ambri board member. Ambri expects to be able to begin full-volume production having raised just over $50 million to date.

In the past, Ambri executives have said they were targeting a battery cost well below $500 per kilowatt-hour. At that price, excess wind energy could be cost-effectively stored at night for sale during the day, or could help utilities meet demand during peak hours. The company’s small-scale commercial prototypes, which are about a cubic meter in size, will be installed at a few locations, including a wind farm in Hawaii and a military base in Cape Cod powered partially by stored renewable energy.

Demand for new battery technologies is especially strong in California and New York, which have programs to encourage installation of energy storage to improve grid reliability. But Chung predicts that many companies testing new products will fail as they miss performance targets or starve for lack of capital from investors. “Many are incremental solutions and are not improving on cost that much,” he says.